1. Field of the Invention
The present invention relates in general to a method of forming a peripheral portion of a blank in the form of a circular plate so as to increase the thickness of the peripheral portion, and more particularly to such a method which permits desired thickening of the peripheral portion with comparatively easy control of a swaging force acting between the peripheral portion and a roller die, while avoiding buckling or folding of the peripheral portion.
2. Discussion of the Related Art
Automotive vehicles use a drive plate such as a flywheel connected to a crankshaft of an engine. Conventionally, such a drive plate consists of a circular plate and a ring gear fixedly mounted on the circumferential surface of the circular plate. To meet a recent need for further reduction of the cost of manufacture of the drive plate, there has been proposed an integral or one-piece type drive plate consisting of a circular plate whose outer peripheral portion is formed into an integral toothed portion. For increased shock or impact resistance and mechanical strength, it is desired that the drive plate be formed to have a larger thickness at its peripheral portion than at its radially inner portion. An example of a method to thicken the peripheral portion of a blank in the form of a circular plate uses a rotary swaging apparatus which includes (a) a rotatably supported roller die having an annular forming groove which is open in its outer circumferential surface and substantially V-shaped in cross section in a plane including an axis Si of rotation of the roller die such that an axial dimension of the forming groove as measured in an axial direction of the roller die increases in a radially outward direction of the roller die, (b) a blank holder jig which is rotated about an axis S2 of rotation thereof and which holds the blank such that the blank is coaxial with the blank holder jig and has a peripheral portion which extends radially outwardly from the blank holder jig over a predetermined radial distance, and (c) a drive device for rotating the roller die and the blank holder jig about their respective axes S1 and S2, such that the blank is held pressed at its outer circumferential surface, in rolling contact with a bottom surface of the forming groove of the roller die, so that the peripheral portion of the blank is thickened following the cross sectional shape of the forming groove.
JP-A-9-10885 shows such a method, wherein the axes S1, S2 of rotation of the roller die and the blank holder jig are parallel with each other, and the blank is held by the blank holder jig such that the blank is subsonically aligned with the forming groove in the roller die, in the axial direction of the roller die and the blank holder jig, and the roller die and the blank holder jig are moved toward each other in the radial direction so that the peripheral portion of the blank 104 is swaged into a thickened portion 100 as shown in FIG. 6A. The thickened portion 100 is thickened substantially symmetrically on both of its upper and lower sides, to have an arrow shape in cross section in a plane parallel to a direction of thickness of the blank. The peripheral portion of the blank is swaged in two steps into the thickened portion 100, which is then bent or folded almost at right angles, by a roller die 102, so that the blank has a cylindrical peripheral portion 108 which extends from one of the opposite surfaces of a radially inner flat portion 106 of the blank 104 in the axial direction, as shown in FIGS. 6A-6C.
In the conventional method of swaging the peripheral portion of the blank, the peripheral portion of the blank must be forced against the bottom surface of the forming groove of the roller die so that the peripheral portion is thickened substantially symmetrically on the opposite sides of the blank while the blank is held substantially flat, except the radially outermost part being thickened. However, the peripheral portion of the blank tends to buckle soon after the force acting between the blank and the roller die in the radial direction of the roller die exceeds a given critical value. It is considerably difficult to control this radial force so as to thicken the peripheral portion without buckling. Further, the thickened peripheral portion 100 which has been formed by the swaging described above is subsequently bent or folded about 90.degree. by the roller die 102 to form the cylindrical peripheral portion 108, as described above. Accordingly, the cylindrical portion 108 suffers from undesirable folds (hairpin bend) as indicated at A in FIG. 6C. These folds A adversely influence a subsequent forging or machining operation on the cylindrical peripheral portion 108 to form a toothed portion. Namely, the formed toothed portion may not have sufficiently high degrees of durability and mechanical strength, during use of a drive plate having the toothed portion at its periphery.
JP-A-9-66330 shows another method of thickening the peripheral portion of a circular plate blank, wherein the peripheral portion of the blank is brought into sliding contact with one of opposed side surfaces which partially define a forming groove formed in a roller die, so that the outer circumferential surface of the peripheral portion is brought into abutting contact with the other side surface of the forming groove, so as to bend the peripheral portion into a substantially L-shaped thickened peripheral portion. This method also suffers from buckling of the peripheral portion of the blank toward the above-indicated other side surface of the forming groove when the force acting between the blank and the roller die exceeds a critical value. In this method, too, the force is difficult to control for intended thickening of the peripheral portion of the blank without buckling. If the thickening operation is continued after the buckling, the peripheral portion of the blank has a meandering or alternating flow of the material, as indicated at fM in FIG. 7 during the thickening operation, causing a fold between alternate curves of the material flow. It is noted that FIG. 7 corresponds to the cross sectional view of FIG. 6C. The alternating flow of the material is generated by initial buckling of the peripheral portion of the blank from the above-indicated one side surface of the forming groove toward the other side surface, and subsequent buckling of the peripheral portion in the reverse direction toward the above-indicated one side surface, as a reaction of the initial buckling. If an angle .beta. one side surface 110 of a forming groove with respect to the radial direction of the roller die is larger than 15.degree., as indicated in FIG. 8, a blank 112 in the form of a circular plate is buckled at its peripheral portion, as indicated at B in FIG. 8, toward the other side surface 114 of the forming groove, without thickening of the peripheral portion. In this case, the thickening operation is restricted.